1. Field of the Invention
The present invention relates to a heart pacemaker, and in particular to a heart pacemaker comprising means for detecting a pacemaker-mediated tachycardia.
2. Description of the Prior Art
Atrial synchronized pacing systems include an atrial (P-wave) sensing circuit, which, in connection with retrograde (ventriculoatrial, VA) heart tissue electrical conduction might cause a so called pacemaker-mediated tachycardia (PMT). A PMT results when the atrial sensing circuit detects a P-wave induced by e.g. a retrogradely conducted ventricular activity outside the sensing circuit's refractory periods and the pacemaker subsequently initiates a paced ventricular beat. Repeated stimulation is sustained by heart tissue retrograde conduction and by pacemaker antegrade conduction.
Although this complication is related to atrial synchronized systems in general, it can be exemplified with reference to dual chamber (DDD) pacemakers. With the advent of the first generation of such pacemakers with relatively short atrial refractory periods, PMT was identified as a significant problem, and later generations of such pacemakers therefore include methods for preventing PMT.
One such known method involves the use of programmable atrial refractory periods, where the atrial refractory period is programmed to be longer than the retrograde conduction interval. However, this method lowers the upper synchronized rate limit of the pacemaker and, in patients with a long retrograde conduction interval, the refractory period required is so long that the advantages of dual chamber pacing are seriously affected. Another known method is based on the fact that the majority of PMT is initiated by ventricular premature beats (VPB), viz. a ventricular event not preceded by an atrial beat. Therefore, this known method foresees that only a VPB triggers a prolonged atrial refractory period. Alternatively, a VPB could trigger a simultaneous atrial stimulation causing the atrium to be refractory when the retrograde conduction occurs. However, although this method generally allows shorter refractory periods, PMT remains a problem in patients where the initiating mechanism for PMT is unknown or is different from a VPB (Spontaneous Endless Loop Tachycardia by Oseran et al, PACE vol. 9, May-June 1986, pages 379 to 386).
Further, it is known that a PMT, once established, can for instance be terminated by omitting ventricular beats or delivering critically timed ventricular premature beats. Here, however, PMT detection is critical, and a possible detection criterion such as a stable high frequency at, or in the vicinity of, the upper rate pacing limit cannot be reliably related to a PMT (Merits of Various Antipacemaker Circus Movement Tachycardia Features by den Dulk et al, PACE, vol. 9, November-December, Part II 1986).